It would be graded on the quality of the flowers, how well they fit with the rest of the scene, and how the flowers are brought out. Anything would basically go otherwise, but it would be graded on those three things. Any kind of flowering plant would qualify.

Something I was thinking of -- it requires post-production though so I don't know if it is fair towards people without access to Photoshop or a similar program (although there are great free ones out there as well!)

My idea was to model/texture/render an object in Anim8or and to incorporate it into a photo as good as possible. Also creativity comes into play to make something interesting.